I set it to whatever the door sticker says. When I worked at Sears Auto people would get so mad at me when they saw me opening their trunk on a tire rotation. I would always set spare tire pressure and almost every one was substantially lower than not only the door psi, but were usually under 20psi.

My VW GTI is a bit rare because it came with a full size V speed rated spare from the factory. I set mine to 38 and check it once a month. Super easy to check. It's not upside down and there is a holder to keep the trunk mat up out of your way.